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Sulfonyl esters. 2. CS cleavage in some substitution reactions of nitrobenzenesulfonatesJ A M E S L A Y TO N A U M , IMAT O LH A SSA N , I CH A RD RANCISA N G L E R , ' AUL OSEPH U J O L ,

A N D R A J K U M A R A H E J ADepartment of Chemistry, Florida Institute of Technology, Melbourne, FL 32901-69 88, U. S.A.

Received May 23, 1989'JAMES LAYTONA U M, I MA T O LH A SSA N ,ICHARDRA N CI SA N G L E R ,A U L O SEPH U JO L, nd RA J K U M A R A H EJA .Can. J. Chem. 68, 1450 (1990).An attem pt to explore arom atic sulfonate esters as agents for the condensation of alcohols with m ercaptans revealed an unusualprocess for sulfonate esters: CS bond rupture. Two m echanistic possibilities for CS bond rupture are explored: (i ) radical anionintermediacy via single electron transfer and (ii) nucleophilic aromatic substitution. Both experiments and molecular orbitalcomputations are presented to support the conclusion that nucleophilic aromatic substitutions are occurring.Key words: sulfonyl esters, nitrobenzenesulfonates, CS bond rupture.JAMES LAYTONA U M , I MA T O LH A SSA N ,ICHARDRA N CI SA N G LER,AUL OSEPH UJOL t RAJ KUM AR AHEJA .Can. J. Chem. 68 , 1450 (1990).Lorsqu'on a essay6 d'utiliser les esters d'acides sulfoniques aroma tiques comme agents de condensation des alcools avec lesmercaptans, on a observe une reaction inusitee pour ce type de comp ost , B savoir une rupture de la liaison CS. On a explore deuxpossibilites mtcanistiques pour expliquer ces scissions : (i) un in termtd iaire radical-anion se formant par le transfert d'un seulelectron et (ii) une reaction de substitution aromatique nuclCophile aromatique. On presente des experiences a insi que des calculsd'orbitales moltcu laires qui sugge rent que les rCactions se produisent par le biais de substitutions aromatiques nuclCophiles.Mots c l b : esters d'acides sulfoniques, nitrobenzknesulfonates, rupture de la liaison CS. [Traduit par la revue]

IntroductionW e r ecently r epor ted ( 1 ) tha t phenyl me thanesu lf ona te 131 O ~ N ~ S O ~ O P ~CH~--@SH +induces the condensa t ion of p - to ly l mer captan w i th pr imar ya lcohols , a s show n in eq . [ I ] . Fur ther s tudy of th i s chemis t r y 2 NaHle d t o t h e o bs er va ti on t ha t p -n it ro p he n yl m et ha ne su lfo na te C H 3 C H 2 0H HM PA o ~ N - - @ s - - @ - c Hcondensed w i th the mer captan leav ing the a lcohol l a r ge lyunused ( eq . [2]). 1 (63%)

[ I PhO S02CH 3+C H 3*H + CH I CH I O H [ 4 ] P h 2 C H S 0 2 0 A r + Ph 2CHOAr + SO 2

2 NaH----+MPA C H ~ - ~ & S C H ~ C H ~ [5] o ~ N ~ s o ~ O P ~C H 3 + s ~

2 Na HC H 3 C H 20 H H MPA CH~*-S-@NO~1

I n an a t t empt to examine n i t r o- subs t i tu ted su l f ona tes tha tmight induce a lcohol- mer captan condensa t ion , a s tudy of thechemistry of a l inkage isomer ic ni trophenyl sulfonate 2 w a sunder taken.

Results and discussionReaction of 2 w ith e thanol and p- to ly l mer captan f ur n ishedthe surpr is ing resul ts shown in eq. [3] . her e appear s to be on lyo n e e x a m p l e o f a sulfonate es ter subst i tut ion react ion leading t oCS bond rupture (2) ( s ee eq . [ 4 ] ) . The r e a r e , how ever , s ever a lknow n examples of CS bond r up tur e in r eac t ions of r e la ted' ~ u t h o ro whom correspondence may be addressed at the Depart-ment of Chemistry, Mount Allison University, Sackville, N.B.,Canada EOA 3C0 .2 ~ e v i s i o neceived April 1 8, 1990.

NaH--tMPA 1 + C H ~ G - S - ~ S O

sulfonyl-containing s tructures (3 , 4) . Repeti t ion of the eqreaction in the ab sence of e thanol perm it ted the isolat ionin the same y ie ld , a long w i th a modes t y ie ld of 3. Trole for e thanol in the formation of 1 can be d i smis sedconsiderat ion.The unexpec ted CS r up tur e in the r eac t ions of 2mer capt ide an ions pr om pted us to examine n i t roa r y l su l fto e s tab l i sh th e r e la t ive pr opens i t ie s f or CS and CO r up turebehaviour of the ideal dini trosulfonate es ter 4 w as m ow i th the a id of the l inkag e i somer ic d in i t rosu l fona tes 5 a an

T he react ions of these d ini trosulfonate es ters with pm e r c a p ti d e a n i o n s i n H M P A a r e d e p ic t e d b e lo w .A ssuming tha t me thyl subs t i tu t ion d ive r t s x% of prf or mat ion to the o the r r ing , a s imp le calculat ion on the rese q s . [6]an d [7 ] leads to the con clus ion that th e ratio of the of 1 f or med by CS r up tur e to the y ie lds of 1 f o r m e d br uptur e w ould b e ca . 19:1 in the hypothet ical react ion ofp- to ly l m er capt ide an ions !T w o r e a so n a b l e m e c h a n i sm s m a y b e a d v an c ed t o a c

Rimed in Canada 1 Irnprirntau Canada

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for the surprisingly facile CS bond rupture observed for thereactions of the bis(nitroary1) sulfonates 5 a , 5 b . Reaction mightproceed through a Meisenheimer complex (A ) as shown inScheme 1. ~lte rnat ivelv,he merca~tideanion ould transfer anelectron to the sulfonate ester, converting it into the correspond-ing radical anion as depicted in Scheme 2.T O explore scheme 1, semi-empirical molecular orbitalcomputations were carried out on the linkage isomers 5a and5 b . The ZINDO calculated LUMOs, presented in Fig. 1,indicate that nucleophilic attack should occur preferentially onthe sulfonyl-substituted phenyl ring in accord with experiment.Given that hard-soft acid-base arguments ( 5 ) militateagainst nucleophilic attack by mercaptide anions (soft) at nitro-

gen or sulfur (hard), the computations suggest a straightforwardchoice between soft acid sites: nitro-bearing carbon andsulfonyl-bearing carbon. Sulfonyl-bearing carbon has the largerLUMO coefficient, indicating that attack there will lead toa more strongly stabilized transition state (5). This analysisshould also satisfactorily rationalize the results portrayed ineq. [51.In contrast to the reaction of 5 a , 5 b undergoes a modestamount of nucleophilic attack on the nitrophenoxy ring. This

may be rationalized by an examination of the NLUMO of 5 6(6). The preferred soft acid site on that ring is the oxygen-bearing carbon (NLUMO coefficient: -0.319) rather than thenitro-bearing carbon (NLUMO coefficient: -0.176). The com-puted difference in energies between the NLUMO and theLUMO for 5a is 0.443 eV. The corresponding value for 5 b is0.279 eV, suggesting a relative enhancement for the role of theNLUMO in the chemistry of 5 b .ZINDO calculations were ~erformed n 4 and its radicalanion 4;. These computations were intended to reveal thenature and propensities of the radical anion in order to assess theviability of the Scheme 2 mechanism. Figure 2 presents selectedbond length changes computed for the conversion 4 + 4' .The greatest contractions in bond length are computed forthose bonds which are actually ruptured during one or moreof the reactions reported herein. d n this basisthe Scheme 2mechanism seems improbable.It is our view that the substitution reactions reported ineqs. [3], [6], and [7] proceed through Meisenheimer complexesai portrayed in the Scheme 1 mechanism. However, arylsulfonates can be drawn into electron transfer reactions (7, 8) .

1 6Relative yield: ( I 1%) Relative yield: (89%)

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CAN. J. CHEM. OL . 68, 1990

Single electrontransfer W

FIG. 1. ZINDO calculated lowest unoccupied molecular orbitals for5a and 5b .

FIG.2. Some calculated bond length changes (A ) induced by conver-sion of 4 into 4'.

Equation [8] presents a final nitroaryl sulfonate ester -mt id e a nio n r e a ~ t i o n . ~In the eq. [8] reaction, it seems extremely likely tmesylate group is replaced first so that both products arthe intermediacy of 10. The formation of 9 is an appreaction for a SET mechanism similar to the one outlScheme 2.In conclusion, it appears that nitroaromatic compounbe involved in reactions proceeding through radical or in reactions proceeding through M eisenheimer comIntroduction of a sulfon ate linkage onto a nitro-bearingring appears to shift pathway preference toward nuclearomatic substitution.

Computational methodsAll computations were performed using the ZIND O mlar orbital method (9). The ZIN DO program is availableof the QUIPU m olecular modelling package from the QuTheory Project, University of Florida.The ground state geometries were obtained at the INlevel, including d-orbitals on sulfur and using a BF GS uHessian search with Newton-Raphson optimization. Ttricted Hartree-Fock method was used for the closeneutral molecule. F or the radical anion (open shell) systunrestricted Hartree-Fock calculation with annihilatioemployed. The INDOIS option was employed to detthe HOMO and LUMO eigenvalues and eigenvectorsMatago-Nishimoto repulsion integrals and fixed groungeometries.3 ~ i n c ehis reaction was conducted early in our research pan equiva lent of ethanol was present to check for p-tolyl ethyformation.

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ZINDO ca lcu la ted s t r uc tur a l pa r amete r s f o r d imethyl su l f oneare c lose r to exper imenta l va lues than those ob ta ined b y 3-21Gcomputa t ions (10). ZI N D O spec t r oscopic pr ed ic t ions agr eew el l w i th exp er imenta l ly de te r mined u l t r av io le t spec t r a on aseries of f ive subs t i tu ted phenyl me thanesu l f ona tes . Z I N D Ospec t r oscopic pr ed ic tions w er e sys temat ica l ly super ior to thoseo b t a in e d b y C N D O / U V c o m p u t a ti o n s .

ExperimentalGeneralThe ir spectra were recorded on a Perkin-Elmer 237B gratingspectrophotometer. Ultraviolet spectra were run on a Beckman DU-2s&ctrophotometer. The nmr spectra were obtained on a VarianEM360A instrument using TMS as the internal standard. Meltingpoints were determined on a Gallenkamp MFB-595 capillary meltingpoint apparatus and are uncorrected.Preparatio n of phenyl p-nitrobenzenesulfonate 2

A solution of phenol (0.43 g , 4.5 m mol) and triethylamine (0.45 g,4.5 mmol) in pyridine (50 mL) was cooled with an ice/water bath.p-Nitrobenzenesulfonyl chlorid e (1.0 g, 4.5 mmo l) was added in smallportions. The ice/water bath was removed and the reaction mixturestirred overnight.Chloroform extraction followed by acid washing gave crude sulfonateester (0.98 g). The crude was recrystallized from methanol affordingcolorless crystals (0.67 g, 2.4 mmol, 54 % , mp 118.1-118.6"C). Theproduct had Rf 0.66 on analytical tlc plates when developed withchloroform; ir (CHC13): 1540, 1390, 1355, and 1150 cm-I; nmr(CDCl3) 8: 8.36 (2H, d, J = 8 Hz), 8.06 (2H, d, J = 8 Hz), 7.30(3H, m), and 6.96 (2H, m). Anal. calcd. for CI2H9NO5S:C 51.60,H 3.24; found: C 51.58, H 3.22.Reaction of 2 with ethoxide and p-tolyl mercaptide anion sSodium hydride (0.65 g, 27 mmol) was suspended in H M P A ~(30 mL). A solution of absolute ethanol (0.63 g, 13 mmol) in HMPA(10 mL) was added dropwise over 5 min. The reaction mixture wasstirred at ambient temperature for 5 min. A solution of p-tolylmercaptan (1.71 g, 13 mmol) in HMPA (10 mL) was added dropwiseover 10 min. T he reaction mixture was stirred at ambient temperaturefor 5 min. The nitrosulfonate 2 (3.82 g, 13 mmol) was added and thereaction mixture stirred at ambient temperature for 2 h. After about10 min the homogeneous reaction mixture was rust colored.The reaction mixture was poured into water (400 mL) containing10% HCl (5 mL). The resultant mixture was extracted with diethylether (three 100-m L aliquots). The wet organic layer was concentrated.The residue was added to water (100 mL) and the resultant mixture

4~~~~ is hexamethylphosphoramide [(CH 3)2 N]3 P0. HMPA istoxic and carcinogen ic (11).

washed with diethyl ether (three 100-m L aliquots). The combined eth erlayers were extracted with 2.5% NaOH (two 100-mL portions). Theorganic layer was dried ( M gS 04 ) and concentrated, yielding crudep-nitrophenyl p-tolyl sulfide 1 (3.79 g).The crude was recrystallized from methanol, affording impurenitrosulfide 1 (2.71 g). The recrystallized nitrosulfide was chromato-graphed on silica gel (250 g) employing carbon tetrachloride elution(100-mL fraction s). Fractions 11 -1 8 were combined and concen trated,furnishing clean nitrosulfide 1 (2.130 g, 8.6 mmol, 63%). Afterrecrystallization from me thano l, the chromatographed nitrosulfide wasshown to be identical to previously obtained material (2) by ir, nrnr,tlc, mp, and mixture mp.Reac tion of 2 with p-tolyl mercaptide anio nsSodium hydride (0.36 g, 15 mmol), p-tolyl mercaptan (1.73 g,13 mm ol), and the nitrosulfonate 2 (3.82 g , 13 mmol) were reacted inHMPA (40 mL ) as above.The crude was subjected to column chromatography on silica gel(400 g) employing hexanes, carbon tetrachloride, and chlorofom assequential eluants. Nitrosulfide 1 (2.23 g , 9.1 mmol, 66.7%) wasobtained along with a sulfonate ester mixture (1.67 g). T he mixturewas chromatographed on silica gel (200 g) employing 1:l carbontetrachloride/chloroform elution. Sulfide-sulfonate ester 3 (0.78 g,2.1 mmol, 16%) was obtained. After recrystallization from methanol,the product had mp 52.3-53.SC, R f, 0.75 upon development with 1 lchloroform/carbon tetrachloride; ir (CHC13); 1380 and 1180 cm- 'nmr (CDC13) 8: 7 .6 (2H, d , J = 9 Hz), 7.25 (6H, m), and 2.40 (3H, s);ms m/e: 356 (M, 33.9% ), 263 (26% ), 199 (100%), and 184 (64%).Anal. calcd. for CI9Hl6o3S2:C 64.02, H 4.52; found: C 64.15,H 4.3 6. Fractions 13-19 were combined and concentrated, givingunchanged nitrosulfonate 2 (0.47 g).Preparation of the dinitrosulfonate ester 5 ap-Nitro-m-me thylphen ol(0.69 g , 4.5 mmol), triethylamine (0.45 g,4.5 mm ol), and p-nitrobenzenesulfonyl chloride (0.99 g, 4 .5 mmol) inpyridine (50 mL), reacted as above, g ave recrystallized 5a (0.85 g,2.5 mm ol, 56% , mp 131.4-132.2"C). The product had Rf 0.25 onanalytical tlc plates when developed with 1: 1 carbon tetrachloride/chloroform; ir (CHC13): 1540, 1395, 1350, and 1190 cm-I; nrnr(CDCI,) 8: 8.40 (2H , d, J = 8 Hz), 8.06 (2H, d, J = 8 Hz), 7.90( l H , d , J = 8 Hz), 6 .96 (2H ), and 2 .35 (3H , s ); ms m/e : 338 ( ~ t ,3.2%), 321 (31 .8%), and 122 (100%). Anal. calcd. forCI3Hl0N207S:C 4 6 . 1 5 , H 2 . 9 7 ; f o u n d : C 4 6 . 1 0 , H 2 . 8 1 .Preparatio n of p-nitro- m -methylphenyl benzenesulfonatep-Nitro-m-methylphenol (5.00 g, 32 mmol), triethylamine (3. 30 g,32 mmol), and benzenesulfonyl chloride (5.75 g, 32 mmol) werereacted in pyridine (50 mL) following the procedure outlined for thepreparation of phenyl p-nitrobenzenesulfonate 2.

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1454 CAN. 1. CHEM. OL . 68. 1990The crude nitrosulfonate was recrystallized from methanol, afford-

ing nitrosulfonate (6.85 g, 23 mmol, 71%, mp 75-76C). The producthad Rf 0.67 on analytical tlc plates developed with 1:l chloroform/carbon tetrachloride; ir (CHC1,): 1525, 1382, 1348, and 1190 cm-' ;nmr (CDC13) 6: 7.40 (8H, m), 2.53 (3H, s); ms m/e: 293 (M t ,0.35%) and 77 (100%). Anal. calcd. for CI3HllNO5S:C 53.23,H 3.78; found: C 53.21, H 3.72.Preparation of p-nitro-m-methylphenyl benzyl sufjide

Sodium hydride (0.41 g, 17 mmol) was added to HMPA (30 mL).A solution of benzyl thiol(2.1 mL, 16 mmol) in HMPA (10 mL) wasadded dropwise over 5 min. p-Nitro-m-methyl-phenylbenzenesulfon-ate (5.00 g, 17 mmol) was added and the reaction mixture stirred atambient temperature for 2 h.

The reaction mixture was subjected to the extraction procedureoutlined under "Reaction of 2 with ethoxide and p-tolyl mercaptideanions".

Crude product was chromatographed on silica gel (400 g) employingcarbon tetrachloride elution followed by chloroform elution. Dibenzyldisulfide (0.63 g) was isolated. The p-nitro-m-methylphenyl benzylsulfide so obtained was recrystallized from methanol, furnishing cleanyellow needles (0.85 g, 3.3 mmol, 20%, mp 58.3-59.2"C). Thenitrosulfide had Rf 0.51 on analytical tlc plates developed with carbontetrachloride; ir (CHCI3): 1525 and 1340 cm-' ; nmr (CDC13)6: 7.86(l H, d), 7.20 (7H, m), 4.16 (2H, s), and 2.53 (3H, s); ms m/e: 259(Mt , 10%) and 91 (100%). Anal. calcd. for Cl4HI3NO2S: 64.84,H 5.05; found: C 64.77, H 4.96.Preparation of p-nitro-m-methyl benzenesulfonyl chloride

p-Nitro-m-methylphenyl benzyl sulfide (3.99 g, 15 mmol) wassuspended in a solution of glacial acetic acid (25 mL) and water (3 mL).Chlorine (ca. 200 mL/min) was bubbled through the mixture for3.75 g . Icelwater cooling was used as required to maintain the reactiontemperature below 30C.

Chloroform extraction followed by base washing gave a cruderesidue. The residue was rectified at reduced pressure, yielding p-nitro-m-methyl benzenesulfonyl chloride (2.73 g, 11 mmol, 75%, bp 142-148"C/1.6 Torr) (1 Torr = 133.3 Pa). p-Nitro-m-methyl benzene-sulfonyl chloride had ir (CHC13): 1540, 1385, 1355, and 1175 cm-' ;nmr (CDCI,) 6: 8.10 (3H, s) and 2.70 (3H, s).Preparation of 5bp-Nitrophenol(0.61 g , 4.3 mmol), triethylarnine (0.47 g, 4.6 mmol),and p-nitro-m-methylbenzene sulfonyl chloride (1.03 g, 4.3 mmol)were reacted in pyridine (50 mL). Procedure and work-up were thesame as those outlined for the preparation of the dinitrosulfonate ester5a .

Recrystallized 5b (0.69 g, 2.0 mmol, 46%, mp 120-121C) hadRf 0.34 on analytical tlc plates developed with 3:2 carbon tetra-chloride/methylene chloride; ir (CHCI,): 1530, 1400, 1350, and1175 cm-'; nmr (CDCI,) 6: 8.30 (2H, d), 7.96 (3H, m), 7.23 (2H, d),and 2.66 (3H, s); ms m/e: 338 (Mf, 27%), 200 (100%), and 136(78%). Anal. calcd. for C13H1&207S: C 46.15, H 2.97; found:C 46.21, H 2.68.Reaction of dinitrosulfonate 5 a with p-tolyl mercaptide anions

Sodium hydride (0.05 g, 2.2 mmol) was added to HMPA (10 mL).A solution of p-tolyl mercaptan (0.29 g , 2 .3 mmol) in HMPA (5 mL)was added dropwise. The dinitrosulfonate 5a (0.75 g, 2.2 mmol) wasadded and the reaction mixture stirred for 2 h. The reaction mixture wassubjected to the extraction procedure outlined under "Reaction of 2with ethoxide and p-tolyl mercaptide anions".

Crude product was chromatographed on silica gel (30 g), affordinga mixture of p-tolyl disulfide and the nitrosulfide 1 (0.21 g). Themixture was rechromatographed on silica gel (25 g), furnishing cleannitrosulfide 1 (0.11 g).Preparation of p-nitro-m-methylphenyl p-tolyl sulfrde6

Sodium hydride (0.32 g, 13 mmol) was suspended in HMPA(30 mL). p-Tolyl mercaptan (1.69 g, 13.7 mmol) in HMPA (10 mL)was added dropwise over 5 min. p-Nitro-m-methylphenyl benzene-

sulfonate (4.00 g, 13.6 mmol) was added and the reaction mstirred at ambient temperature for 2 h.The reaction mixture was subjected to the same extraction proas the 5a mixture.Crude product was chromatographed on silica gel (400 g), afp-tolyl disulfide (0.80 g, 3.2 mmol) and the nitrosulfide 6 (5.8 mmol, 43%, mp 45.6-45.9"C). The nitrosulfide 6 had Rf analytical tlc plates developed with carbon tetrachloride; ir (C1510 and 1340 cm-' ; nmr (CDCI,) 6: 7.86 ( lH , d), 7.16 (62.50 (3H, s), and 2.33 (3H, s); ms m/e: 259 (M t, 100%), 198and 123 (14%). Anal. calcd. for CI4Hl3NO2S: 64.84, H 5.05C 64.71, H 4.98.Reaction of dinitrosulfonate 5b with p-tolyl mercaptide anion

Sodium hydride (0.05 g, 2.3 mmol) was suspended in (10 mL). A solution of p-tolyl mercaptan (0.26 g, 2.1 mmol) in(5 mL) was added dropwise over 1 min. The dinitrosulfonate e(0.70 g, 2.0 mmol) was added and the reaction mixture stiambient temperature for 2 h .

The reaction mixture was subjected to the same extraction proas the 5 a and 6 mixtures.

The crude product was chromatographed on silica gel affording p-tolyl disulfide (0.13 g, 0.5 mmol) and a mixture conp-nitro-m-methylphenyl p-tolyl sulfide 6 (0.07 g) and p-nitrop-tolyl sulfide 1 (0.009 g). The sulfide mixture was homogenetlc. The sample was split in half. The presence of each sulficonfirmed by the addition of 15-mg aliquots of each sulfideinduced the expected changes for appropriate signals in the nmrof the mixtures.Preparation of o-dibrorno-p-nitrophenyl methanesulfonate 7o-Dibromo-p-nitrophenol(4.98g, 16 mmol), triethylamine (16 mmol), and methanesulfonyl chloride (1.93 g, 16 mmoreacted in dry pyridine (25 mL). Procedure and work-up were thas those outlined under "Preparation of the dinitrosulfonate estRecrystallized o-dibromo-p-nitrophenyl methanesulfonate (10 mmol, 61%, mp 135-136C) had Rf 0.77 on analytical tlcdeveloped with chloroform; ir (CHCl,): 1540, 1390, 1351190 cm-I; uv (CH30H) A,,,: 385 (s, e 172), 270 (e 735224 (e 16 176) nm; nmr (CDC13) 6: 8.46 (2H, s) and 3.56 (Anal. calcd. for C7H5Br2N05S:C 22.42, H 1.34; found: CH 1.30.Reaction of 7 with ethoxide and p-tolyl mercaptide anions

Sodium hydride (0.66 g, 27 mmol) was added to HMPA (3A solution of ethanol (0.62 g, 13 mmol) in HMPA (10 mL) wadropwise over 5 min. Upon completion of the addition, a solp-tolyl mercaptan (1.71 g, 13 rnmol) in HMPA (10 mL) wasdropwise over 5 min. The nitrodibromomesylate (5.24 g, 14was added and the reaction mixture stirred at ambient tempera2 h. The reaction color changed from khaki to red, then to yellofinally orange within the first 15 min. The orange color persistthe end of the reaction.

The reaction mixture was subjected to the extraction prooutlined under "Reaction of 2 with ethoxide and p-tolyl meranions".

The crude residue was twice chromatographed on siliaffording pale yellow crystals of 3,4,5-tris(thio-p-tolyl)nitro8 (0.73 g, 1.4 mmol, 32%). Upon recrystallization from mthe product had mp 144-144SC, Rf 0.40 on analytical tldeveloped with carbon tetrachloride;ir (CHCI3): 1520 and 134uv (CH30H) A 395 (s, e 2060), 375 (e 2681), 253 (e 5666207 (e 157 575);nmr (CDC1,) 6: 7.2 (14H, m), 2.43 (6H, s), a(3H, s). Anal. calcd. for C27H23N02S3: 66.22, H 4.73;C 66.30, H 4.60.

In addition, colorless crystals of 2,6-dibromo-1 4-bis(thiobenzene 9 (0.76 g , 1.5 rnmol, 23%) were obtained. Upon recrytion from methanol the product had mp 93.5-94"C, Rf 0analytical tlc plates developed with hexanes; ir (CHC13): 11500 cm-'; uv (CH30H) Am : 300 (e 9070), 275 (E 18 50

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BAUM ET AL. 145522 0 ( e 31 111); nmr (CDC13) 6: 7 .20 (lOH , rn), 2.33 (3 H, s), and 2.23 4 . ( a ) H . 0 .FO N G ,W. R. HARDSTAFF,. G . K A Y , . F. LA N G LER,(3H, s). Anal. calcd. for C20H1 6Br2SZ: 50.01 , H 3.35; found: R. H. MOR SE, nd D. N. SA N D O V A L.an. J. Chern. 57, 1206C 49.95, H 3.33. (1979); (b) J. F. KING nd K. C. KHEMANI. an. J. Chern. 63,619 (1985).,

Acknowledgements 5. I. FL EM ING . rontier orbitals and. organic chem ical reactions.Wiley, New York. 1976.The au thor s acknow ledge the gu idance of on e of the r ef e rees 6 . ( a ) G . K LO PMA N .. Am. Chern. Soc. 90, 223 (1968); (b)in pr oduc ing the f ina l f o r m of the exper imenta l pa r t o f th i s L. SALEM . . Am. Chem. So c. 90, 543 (1968); 90, 55 3 (1968):manuscr ip t . P r of es sor M . Zem er has k ind ly pr ovided acces s to 7. K. A . DURKIN, . F . LANGL ER,nd N . A. MORRISON.an. J.th e ZINDO pr ogr am u t i li zed in the co mputa t iona l pa rt o f th i s Chem. 66, 3070 (1988).s tudy . 8 . J . C . C A R N A H A N ,R., W. D. CLOSSON,. R . GANSON , . A.JUCKETT,nd K . S . Q U A A L.. Am. Chern. Soc. 98 ,2526 (1976).9. J. RIDLEY nd M. ZERN E R.heor. Chim. Acta, 32, 111 (1973).1. R . F . LA N G LERnd N . A. MORRISON. an. J . Chem. 65 , 2385 10. W. J . PIETRO,M. M. FRANCL,W. T. HEHRE, . J . DEFREES,(1987). J. A. POPLE, nd J . S . BI N K LEY .. Am. C hern. Soc. 104, 50392. J. F. KING and M. ASLA M. an. J. Chem. 57, 3278 (1979). (1982).3. (a ) B. A. KEN T nd S. SMILES. . Chem. Soc. 422 (1934); (b) 11. N. I. S A X .Dangerous properties of industrial materials. VanW. J. EVANSnd S. SM ILES. . Chem. Soc. 181 (1935). Nostrand Rheinho ld, New York. 1979. p. 721.